A fluorescence spectrophotometer generally includes a light source, an excitation monochromator (or a primary monochromator), a sample holding cell, an emission monochromator (or a secondary monochromator), a photometer and a signal processing circuit. An appropriate monochromatic light is selected by the excitation monochromator from the light generated by the light source, and the selected monochromatic light is irradiated onto a sample in the sample cell. There fluorescent light specific to the sample is generated in response to the irradiation. The fluorescent light is led to the emission monochromator, where a component light having a certain wavelength is selected. The strength of the selected fluorescent light is measured by the photometer. The measured strength is referred to an analytical curve to determine the content of a component of the sample. The analytical curve is prepared beforehand with standard samples having various known contents.
In a measurement of a fluorescence spectrophotometer, the output of the photometer is generally passed through a lowpass filter before using the output in determining the content value in order to eliminate noises from the output signal.
The lowpass filter is characterized by the response. When the lowpass filter is constructed by an analog circuit including a capacitor or an inductance, the response corresponds to the time constant .tau. of the lowpass filter. Of course, the lowpass filter can be constructed by a digital signal processing circuit (digital filter) or by a microcomputer with an appropriate filter program.
Noises in the output signal of the photometer are eliminated more effectively as the response value is set larger. But, as the response value becomes larger, the measurement time becomes longer. When the strength of the output signal of the photometer is large enough, it is unnecessary to set a large response value since the strength of the noise is relatively small (i.e., S/N ratio of the output signal is large enough). On the other hand, if the response value is set too small, the output signal of the photometer fluctuates and the content value of the sample determined from such output signal lacks reliability and accuracy.
Some conventional fluorescence spectrophotometers have an automatic response changing function. In those fluorescence spectrophotometers, however, the response is changed according to the wavelength scanning speed of the emission or excitation monochromator in order to prevent shift of the spectrum peak. When the fluorescence spectrophotometer is used in a quantitative measurement, the response is determined manually.